1. Field of the Invention
The present invention relates generally to a stainless steel having high oxidation resistance. More specifically, the invention relates to a Fe-Cr-Al alloy having satisfactorily high oxidation resistance and spalling resistance. Further particularly, the invention relates to a Fe-Cr-Al alloy suitable for a catalyst substrate of a catalytic converter.
2. Description of the Background Art
In recent years, atmospheric pollution due to existance of NO.sub.x, CO and so forth has become a serious social problem. Such atmospheric pollution is led by exhaust gas from combustion facilities, such as internal combustion engines, boilers and so forth. Especially, pollution control has grown as one of the most important tasks to be achieved in the automotive vehicle technology. Therefore, it has become common to provide catalytic converters in exhaust systems of the automotive internal combustion engines.
As is well known, the catalytic converter generally comprises a catalyst substrate made of a ceramic and catalyst coated on the catalyst substrate surface. The catalyst is held on the catalyst substrate surface by means of a catalyst carrier. Conventionally, cordierite (2M.sub.g O0.2Al.sub.2 O.sub.3 0.5SiO.sub.2) has been utilized as a material for forming the catalyst substrate. In the typical construction, the cordylite catalyst substrate is formed into honeycomb structure by extrusion and baking. .UPSILON.-alumina fine particles are coated on the surface of the cordierite catalyst substrate to serve as the catalyst carrier. A catalyst made of platinium (Pt) and so forth is bonded on the catalyst carrier.
Another catalytic converter has been disclosed in the U.S. Pat. No. 4,331,631, issued on May 25, 1982, to Chapman et al. The disclosure suggests replacing the cordierite catalyst substrate with a metal substrate assembled by an oxidation resistant stainless steel foil into honeycomb structure. By replacing the cordierite catalyst substrate with the thin stainless steel foil catalyst substrate, the wall thickness of the honeycomb structure becomes thinner to expand the open air ratio of the honeycomb. As a result, the path area for the exhaust gas can be expanded. Since such catalyst substrate may provide a wider path area for the exhaust gas passing therethrough, the back pressure of exhaust gas can be reduced and good engine performance can be obtained. This, in turn, means that the size of the catalytic converter can be reduced to be compact enough by employing the stainless steel foil catalyst base.
As is well known, the catalyst carrier is held on the surface of an oxide layer formed on metal substrate. It is important that the alloy used as the substrate has good oxidation resistance and spalling resistance.
The disclosed invention employs Fe-Cr-Al alloy added an yttrium (Y). In the disclosure, the Fe-Cr-Al alloy is composed of chromium (Cr) of 15 to 25 Wt%, aluminum (Al) of 3 to 6 Wt% and Y of 0.3 to 1.0 Wt%. Y is indeed a rare and expensive material. Furthermore, Y cannot be supplied at a sufficient amount for use in the automotive industry to manufacture the catalytic converters.
On the other hand, the U.S. Pat. No. 4,414,023, issued to Aggen et al. on Nov. 8, 1983, discloses a Fe-Cr-Al alloy composed of Cr of 8 to 25 Wt%, Al of 3 to 8 Wt%, and an addition of at least 0.02 Wt% and up to 0.05 Wt% from the group consisting of cerium (Ce), lanthanum (La), neodymium (Nd), praseodymium (Pr) with a total of all rare earth metals (REM) up to 0.06 Wt%. This alloy will be hereafter referred to as "Fe-Cr-Al-REM alloy ". In this Fe-Cr-Al-REM alloy, REMs improve the adherence of the oxide layer. Such alloy has been conventionally used for electric resistance heating elements.
The Fe-Cr-Al-REM alloy has reasonably high oxidation resistance when it is used in a form of a relatively thick plate. However, when it is used as the catalyst substrate, the thickness of the foil has to be thin enough to provide sufficient path area in view of the engine performance as set forth above. If the temperature of the exhaust gas rises when substantially high load is continuously applied to the engine as in high speed cruising, or a spark ignition timing is retarded excessively, rapid oxidation of the overall structure of the alloy occurs and the substrate becomes an oxide which is weak or brittle and tends to be easily broken. In addition, as is also well kown, pulsatile flow of the exhaust gas tends to be generated during engine driving to cause vibration simltaneously with high temperature oxidation. This tends to cause releasing of the oxide scale from the associated surface of the catalyst substrate. As set forth above, since the catalyst is bonded on the oxide scale by means of the catalyst carrier, the releasing of the oxide scale leads to removal of the catalyst to lower the exhaust gas purification performance of the catalytic converter.
It should be noted that, throughout the following disclosure, spalling resistance is used to represent the property of good adherence of the oxide scale on the surface of the catalyst substrate.